This application claims the priority of German patent document 100 31 542.9, filed Jun. 28, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an inertial sensor as an inertial reference for determining the attitude and position of satellites and satellite parts.
A new generation of scientific space missions, particularly based on high-resolution optical instruments, requires a very precise knowledge of the residual acceleration, the relative positions and the attitude as well as attitude fluctuations of satellites and satellite parts. The degrees of freedom associated therewith are influenced to an extent which should not to be neglected, by both internal interfering influences (for example, eccentricity of the center of gravity or change of the moment of inertia) and external interfering influences (for example, solar wind and residual magnetic fields). At the European level, missions which are relevant in this respect are GAIA, IRSI, LISA, DIVA, GOCE and STEP.
So far, acceleration sensors, which are connected with a test mass by way of a soft coupling, have been used to determine the residual acceleration of satellites and satellite parts. For determining the attitude and position, high-resolution optical astral sensors (in the form of, for example, CCD cameras, such as the Hubble telescope) are predominantly used.
Inertial sensors have been used only as an inertial position reference. To determine the position of satellites or satellite parts, the position of a test mass is measured relative to its satellite environment. The position of the test mass is normally determined capacitively and checked by means of electric fields. For this purpose, the generally metallic test mass is electrostatically charged. The electrostatic charging as well as the induced charge polarizations cause forces and moments (for example, Lorentz force by movement in an external magnetic field or electric dipole moments) which interfere with the inertial movement of the test mass. As a result of stray electric fields (parasitic capacitances), particularly in the case of large distances (in the range of several millimeters) between the test mass and the reference surfaces (electrodes), a non-linear behavior of the distance measurement will occur. To minimize the direct effect of the magnetic field on the test mass, a material of low susceptibility is selected for the test mass. Another measure for the shielding with respect to magnetic fields is the use of xcexc-metals; however, although they have a high magnetic permeability, the latter are considerably impaired in their effect as a result of starting vibrations of the satellite.
U.S. Pat. No. 4,170,904 discloses an inertial sensor in which a test mass is situated in a closed housing that shields it from interfering external influences (such as the radiation pressure of the sun, electric static fields). For controlling the attitude and position, the test mass is exposed to a controllable magnetic field generated by means of coils.
German Patent Document 199 21 390 A1 describes a positioning system for a measuring device of a satellite, in which the entire measuring device, including the electronic system or at least parts thereof, is uncoupled from the satellite surrounding the measuring device. Thus, it is possible to uncouple the measuring device from external interferences. However, the high constructional expenditures represent a disadvantage.
In European Patent Document EP 0 569 994 A2, an inertial sensor is described in which a test mass is situated in a space that is free of electric and magnetic fields, and is surrounded by a housing. Optical measuring sections determine the attitude and position of the test mass, which is adjusted by means of low-frequency or high-frequency sound waves emitted from a wave projector and directed against the movement of the test mass. The disadvantage of this arrangement is its cost-intensive construction, as well as the inexact measurement of the attitude and the position of the test mass.
It is an object of the invention to provide an inertial sensor which achieves an improved, highly precise determination of the attitude and position of satellites and satellite parts, and which is not susceptible to external interfering influences.
This and other objects and advantages are achieved by the inertial sensor arrangement according to the invention, in which the optical measuring sections are constructed as optical interferometric measuring sections. The attitude and position of the test mass can be adjusted by means of the pressure of light exerted upon the test mass in the optical interferometric measuring sections. On the one hand, the test mass can inertial float freely in the housing, and ideally can be subjected only to the gravitational interaction during the process. Or, it can conformably with the housing, rotate about a joint axis at an identical nominal angular velocity.
In order to avoid the creation of radiation gradients inside the housing (which may result in accelerations in the range of from 10xe2x88x9215-10xe2x88x9213 m/s2), in an advantageous embodiment the housing enclosing the test mass is thermally constructed as a black body (for example, by insulation). Thus, scattered light, which may have been introduced into the housing by the optical measuring sections, is uniformly thermalized (Ulbricht globe).
The housing should be mechanically stabilized because it contains the reference elements and should consist, for example, of glass ceramics (Zerodur(copyright) or ULE(copyright)). In addition, the housing may be shielded against residual magnetic fields.
On the interior surface of the housing, the housing-side reference elements are situated for the relative attitude and position determination of the test mass. In addition, the interior surface of the housing can be covered with a conductive coating (such as gold) in order to eliminate electrostatic fields. Furthermore, the housing may be evacuated or filled with gas.
In another preferred embodiment, the surface of the test mass can be constructed as a reflector, for example, in a metallized manner. As a particularly advantageous embodiment, optical reflector elements, such as mirrors, can be applied to the surface of the test mass. (These reflector elements may have a planar or spherically centered design.) The surface of the test mass or the reflector elements applied to the surface form an end mirror of one of the optical interferometric measuring sections.
The test mass has an advantageously symmetrical construction and has the shape of a cube, a right parallelepiped, a tetrahedron, a disk or a ball. Its shape depends on the requirements for compactness (minimizing of the residual interfering elements) as well as on the desired moments of inertia in the axes of rotation.
In particular, the shape of the test mass is independent of the selected optical arrangement for determining the attitude and position. The material of the test mass is determined to minimize the interfering influences. Important features for selecting the material are:
A minimal magnetic susceptibility,
a high thermal and electric conductivity,
a high density, and
a low thermal coefficient of expansion.
The determination of the attitude and position of the test mass, along the desired degrees of freedom and relative to the reference elements on the housing, takes place by means of laser-metrological methods that are known per se. Suitable methods are described in the literature [1] E. Morrison, B. J. Meers, D. I. Robertson and H. Wald; xe2x80x9cAutomatic alignment of optical interferometersxe2x80x9d; Applied Optics; Vol. 33; No. 22; 1994; p5041; [2] N. M. Sampas and D. Z. Anderson; xe2x80x9cStabilization of laser beam alignment to an optical resonator by heterodyne detection of off-axis modesxe2x80x9d; Applied Optics; Vol. 29; No. 3; 1990; p394; [3] D. Z. Anderson; xe2x80x9cAlignment of optical cavitiesxe2x80x9d; Applied Optics; Vol. 23; No. 17; 1984; p2944; and [4] B. Hines, M. Colavita, K. Wallace and A. Poulsen; xe2x80x9cSub-nanometer laser metrology-some techniques and modelsxe2x80x9d; Proceeding of high resolution imaging by interferometry 1; Garching; 1991; p1195.
Particularly the respective following measuring methods can be used:
Heterodyne interferometry in the following also called V1;
classic interferometry by means of a Michelson interferometer (V2); and
use of an optical resonator (Fabry-Perot) with a monitoring of the resonator modes by means of heterodyne methods (V3).
By means of laser metrological methods, relative distance measurements can be carried out with a precision around 10 pm at rates of approximately 1 s. The precision of the angle measurements is below 0.1 nrad.
During the measurement, as a result of the optical interferometric measuring sections aligned with the test mass, a pressure of light in the order of 0.0035 xcexcN/W is exerted on the test mass. In a particularly advantageous construction, these optical measuring sections are aligned with respect to one another such that the pressures of light exercised by the individual optical measuring sections upon the test mass compensate one another. In this case, the optical measuring sections are advantageously aligned with the mass center and the geometric center of the test mass.
In a preferred embodiment, the position and attitude of the test mass can be adjusted by variation of the pressure of light in the individual optical measuring sections.
By appropriate adaptation of the resonator in a preferred embodiment of method V3, the position and the attitude of the test mass can be controlled in a precise manner. The precise adjustment of the test mass takes place by the targeted excitation of the longitudinal and transverse resonator modes forming in the resonators, while utilizing the resonance step-up which leads to increased light pressure value while the laser light output is lower (a few mWs). Furthermore, the measurement of the resonator modes permits a precise determination of the attitude and position of the test mass. The high tuning precision of the resonator modes as well as the high quality of the resonators are also found to be advantageous.
The advantageous alignment of the optical interferometric measuring sections as well as the advantageous selection of the laser working frequencies on a flank of a resonator mode permits in method V3 an inherent self-centering of the test mass by the automatically adjusting variation of the light pressure in the resonator modes.
The advantages of laser-metrological methods in comparison to capacitive measuring devices are:
The elimination of the interfering influences of electric fields, whereby a better uncoupling of the satellite body is permitted;
a high resolution into the picometer range;
an extended linear characteristic curve around the working point;
the determination of the attitude and position of the test mass in up to 6 degrees of freedom;
selection of the distance between the test mass and the housing (reference elements), within a wide range (xcexcm to m).
Additional advantages and advantageous embodiments of the invention will be described in the following by means of drawings which illustrate the basic construction of the optical inertial sensor according to the invention in an embodiment A1 and an embodiment A2. A1 is an inertial sensor according to the invention as a position and rotation reference for an inertially resting or slowly rotating satellite; and A2 is an inertial sensor according to the invention as a joint position reference of two or more optical interferometric measuring sections situated at a variable angle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.